div {
    background-color:red;
    width: 500px;
    height: 100px;
}
/*id选择器*/
#first-div {
    background-color:blue;
    width: 400px;
    height: 200px;
}
/*class选择器*/
.bg-div {
    background-color:black;
    width: 300px;
    height: 300px;
}